The present invention relates to an apparatus and a method for controlling a longitudinal shifting of an elongated piece along a predetermined length as the piece moves transversely within a plane at a given speed. According to a preferred application, the present invention relates to a board positioning apparatus and method for precise saw trimming. Such apparatus is generally mounted in front of a multiple saw trimmer.
Known in the art there is a mobile gate system with cylinders. Pneumatic cylinders are used for positioning a mobile gate. Limit switches are used for sensing the position of the gate. The gate is moved to a desired position and a lumber piece is moved against the gate. When the lumber piece is in position, an elongated arm lifts the lumber piece to keep its position for the trimming. One of the drawbacks with this equipment is that the accuracy of the displacement is in relation with the distance between the limit switches. Moreover, the stroke for one cylinder is typically limited to 6xe2x80x3 and for an additional displacement, it is then necessary to add another gate. Such system causes a rebound of the lumber piece on the gate and thus an additional imprecision occurs. The maximum speed of this system is generally 100 lumber pieces per minute.
Still according to the prior art, there is known a pivoting gate system with a cylinder. The operation of this system is similar to that of the mobile gate described above. The rebounds are however avoided since the lumber piece has a given continuation with the gate. The cylinders that are used are hydraulic cylinders with position sensors. One of the drawbacks of this equipment is that it requires an extensive maintenance and a high and expensive hydraulic power. The cylinders are also very expensive. The stroke of a cylinder is limited to 6xe2x80x3 and for an additional displacement, another gate has to be added. The maximum speed of this system is generally limited to 120 lumber pieces per minute.
Also known in the art, there is the Canadian published application no. 2,228,641 in the name of the applicant, wherein there is described an apparatus for positioning lumber pieces. This Canadian application corresponds to U.S. application Ser. No. 60/076,194. The apparatus comprises several levers mounted in rows. These levers are for slightly lifting the lumber pieces and thus preventing them to be longitudinally displaced by means of the aligning rollers. The lumber pieces are then pushed by means of wedges and slid over the levers. The geometry of the levers prevents abrupt movements of the lumber pieces. A drawback with this apparatus is that it is not compact and consequently requires a large space for its installation. As a matter of fact, when such positioning apparatus is mounted in existing plants, other important equipment may have to be removed to provide the necessary space for the installation of the apparatus.
It is an object of the present invention to provide a method and an apparatus that while compact can process a large number of elongated pieces per minute.
According to the present invention, there is provided an apparatus for controlling a longitudinal shifting of an elongated piece along a predetermined length SL as said piece moves transversely within a plane at a given speed PS, comprising: a longitudinal arm having a given length PL pivotable within the plane to shift the piece; a flexible strap operatively mounted around the arm to contact the piece as the arm is pivoted to shift the piece; a controllable pivoting means for pivoting the arm according to an angle xcex8 determined in relation to the predetermined length SL and the given length PL; and a controllable driving means for driving the strap around the arm according to a speed SS determined in relation to the speed PS of the piece and the angle xcex8 so that the longitudinal shifting is controlled.
Also according to the present invention, there is provided a method for controlling a longitudinal shifting of an elongated piece along a predetermined length SL as said piece moves transversely within a plane at a given speed PS, comprising steps of: (a) providing a longitudinal arm having a given length PL pivotable within the plane to shift the piece, and a flexible strap operatively mounted around the arm to contact the piece as the arm is pivoted to shift the piece; (b) pivoting the arm according to an angle xcex8 determined in relation to the predetermined length SL and the given length PL of the arm; and (c) driving the strap around the arm according to a speed SS determined in relation to the speed PS of the piece and the angle xcex8 so that the longitudinal shifting is controlled.